On January 25, 2004, a strange object fell out of the sky on a distant planet—and when it hit the surface, it started to bounce. Even though that airbag-cushioned descent was exactly how things were planned, it wasn't exactly an elegant start to what's turned out to be a record-setting journey for Opportunity, which continues to operate long past its minimal mission time of 90 days.

Opportunity may be notable for its longevity and all the scientific data that it's sent back to Earth over the past decade. But it's also notable in how the rover has set standards for what we expect exploration missions to look like, from the hardware to the media coverage. When the mission is appreciated in its full context, it becomes a story that goes well beyond the little rover that could; it's the story of a mission that ushered NASA into a new era of space exploration.

Big projects on little budgets

Fights over NASA's budget go back for decades. They've mostly focused on the big-ticket item: manned space flight. But as far back as the Mariner and Pioneer programs, NASA was also funding robotic exploration. With the staggering success of the Voyager probes and the failed promise of the Shuttle program—which didn't make going to orbit either cheap or simple—the appeal of robotic exploration was significant.

MERs were one of the first tastes of NASA's future, where the huge, expensive flagship missions will be increasingly rare.

But Voyagers are what NASA now terms a "flagship program": large, complex, and expensive. As budget constraints continued into the indefinite future, it became increasingly clear that flagship missions were going to be few and far between. NASA started getting interested in simpler and less expensive missions that could still deliver quality science. In the early 1990s, NASA leadership established the Discovery Program, intended for scientists outside of NASA who could deliver a mission within a hard budget cap. By 1997, this funding mechanism sent Mars Pathfinder to the red planet's surface, a mission that included a small rover named Sojourner.

Spirit and Opportunity, the twins that made up the Mars Exploration Rovers (MERs), were both a logical and financial progression from Pathfinder. As the second project in NASA's Discovery Program, the MERs were already in the works while Sojourner was on its way to Mars. Given a budget that was held under $500 million per rover and the leadership of Cornell's Steve Squyres (the project's principal investigator), the rovers fit the program's outlines nicely.

What wasn't necessarily clear at the time was that Discovery-class missions weren't just a good way to make due until NASA's budget situation improved; instead, the future of NASA looks to be filled with them. That was driven home by the recent decadal survey, which set the priorities for space exploration for the next ten years. The highest priority flagship mission in it, a landing on Europa, has since been cancelled by NASA. In effect, the MERs were one of the first tastes of NASA's future, where the huge, expensive flagship missions will be increasingly rare.

And that was true in more ways than budget considerations.

It’s how we roll

The tight budgets necessitated some compromises—the MERs have far fewer instruments than the flagship-level Mars Science Laboratory, which recently joined the rovers on the red planet. But both in terms of hardware and strategy, the MERs were a key evolutionary step on the road that took us to the Mars Science Laboratory and the approach to exploration that it offered.

Prior to Mars Pathfinder (and even a bit after), our Martian landers were just that: landers. You had to hope they either landed some place typical so that you got a feel for the more general geology of the body they landed on, or some place interesting, in which case the discoveries would make up for the fact that what you were seeing might not be representative. The worst case scenario, in many ways, was to have a lander set down somewhere near something that looked interesting but which was not close enough to do anything more than stare at it.

The problems with this situation are pretty obvious, and the Sojourner rover showed that this didn't have to be the case; if you landed near something that looked like it was worth checking out, you just went over to take a closer look. Opportunity and its twin were the logical evolution of this approach. Rather than being an extension of the lander, the rovers were explorers—and specialized ones at that. In a 2003 paper (published just weeks before the rovers' landing) that described their capabilities, Squyres' team described them as "robotic tools for conducting field geology on the surface of another planet." To perform that function, they carried the tools geologists might have: stereoscopic cameras to help identify areas of interest, an abrasion tool to get gunk off the surface of rocks, and spectrometers to probe their composition.

And, most importantly, wheels to take them there. Being mobile meant that the rovers weren't limited to what was in reach of the lander; they could go off and look at sites near the lander that looked interesting—and then continue searching for other areas that looked interesting. Even in the absolute low-end estimate of the rover's lifetime, they were expected to travel at least 600 m from the landing site, with expectations of them reaching a kilometer.

And that was the absolutely low-end lifetime estimate; even while they were in transit to Mars, their creators were writing, "Near the end of the mission when output from each rover's solar arrays is diminishing, operations will be changed to conserve electrical power and extend mission duration." The rovers weren't going to live long enough to simply determine if the landing site was interesting or not; they'd head off and hunt down anything interesting and determine whether the landing site was typical of the region or not.

Further Reading

In that, the rovers were aided immensely by the fact that they were part of a larger program intended to study Mars comprehensively. They were preceded to the red planet by the Mars Odyssey orbiter, which could image and determine the average composition of the region the rovers were in. By 2005, Odyssey was joined by the Mars Reconnaissance Orbiter and its spectacular HiRISE camera. Combined, this hardware could provide a clear picture of what was happening on Mars, with the orbiters providing the general overview and the rovers nailing down the details.

Steve Gorevan of Honeybee Robotics, which built the rovers' rock abrasion tool, agreed, telling Ars, "The exploration of Mars has been a step-by-step exploration featuring the installation of an infrastructure on orbit around Mars and on the surface. The payloads are tailored to build on what went before and in part what is to come. The truth on the ground is connected to the truth on orbit and the science is incrementally solid."

And it worked. By 2003, Opportunity's team was writing, "Despite the generally homogeneous appearance of Meridiani Planum from orbit, a number of new features and phenomena have been observed at these locations at rover scale." As Gorevan put it, "The exploration of Mars has been a systematic triumph."

Again, it's hard to view this as anything other than the likely future of exploration. If we really want to know about a body's present and history, a combination of intensive orbital surveys and rovers that provide detailed samples of varying terrain seems like the best way to do it. It's hard to imagine any space program settling for less if it really wants to understand a place.

In for the long haul

But the key to painting that picture has been the rovers' longevity, which has made the guaranteed minimum of 90 days seem laughably conservative. Why have the rovers done so phenomenally well?

The reason for the incredibly low minimum mission profile is two-fold. One is that the rovers had multiple points of failure. Any serious problems with their power system, batteries, electronics, or drive train could cripple them. Some of these potential failures aren't as problematic as others; radiation-hardened electronics are extremely conservative: the MERs' CPUs ran at 20MHz, and their different forms of memory added up to less than a gigabyte (memory issues could potentially be handled by avoiding the faulty section of memory). And while a wheel failure doomed Spirit, the wheels on Opportunity haven't done anything more than squeak. Careful use of the wheels has helped the rover escape small sand dunes on more than one occasion.

That isn't to say that Opportunity has been glitch-free. The rovers are designed to travel with their robotic arms stowed; the motor that re-extends Opportunity's arm nearly failed, and the arm was only liberated with a large jolt of current. Not taking any further chances, the rover's handlers have figured out how to position the arm so that Opportunity can drive with it extended.

Still, even if everything were in completely working order, the rovers were expected to die for a simple reason: power. Instead of relying on heat from the decay of radioactive elements as Curiosity does, the MERs are solar-powered, with lithium batteries to store power when the Sun isn't available. A minimum amount of power is required just to start the rover up, and many parts need to be heated in order to function properly in the cold Martian environment. If power gets too low after a long winter night, the rover might simply be unable to restart, even after the solar panels start producing power.

In the long term, the lithium batteries will eventually doom Opportunity (how many of those are still working well after 10 years of daily use?). But they weren't considered an issue at the start. Instead, Mars' dusty environment was expected to slowly strangle the rovers, covering their solar panels until they failed to recharge the batteries sufficiently. Oddly, it was Mars' dust storms that kept this from happening. Rather than adding to the layer of dust on top of the panels, the winds swept them clean, leading to a large surge in power production once the storms subsided.

That situation, combined with the careful power management that was planned from the start, has been enough to keep Opportunity going for a decade.

Opportunity's travels across the largely flat and sandy Meridiani Planum are punctuated by visits to craters. Here, on its way to Endeavour Crater, it stumbled across the small (10 m) Nereus crater, with lots of exposed rocks nearby.